1. Field of the Invention
The present invention relates to an electric motor for use in an electric vehicle.
2. Description of the Related Art
As the global warming issue is becoming more serious, more attention is paid to electric vehicles which provide less environmental load, and various technologies have been proposed as elementary technologies of electric vehicles. For example, in electric motors (hereinafter, referred to as a motor) for use in electric vehicles, a water-cooled motor has been proposed in which a cooling water jacket is provided on an outer circumference of a stator for circulation of cooling water for cooling the motor in order to maintain the temperature in the interior of a motor in operation to a permissible value.
As a construction of a cooling water jacket in a water-cooled motor, there has been proposed a construction in which a space between an inner housing into which a stator is inserted and an outer housing which configures a cooling water jacket is completely hollowed in order to ensure a required cooling capability. In addition, there has also been proposed a cooling water jacket construction for increasing further the cooling effect in which radiation fins are provided in a hollow portion in a cooling jacket while considering a flow of cooling water (refer to JP-A-8-19218).
The operation speed of motors for electric vehicles reaches about 10000 rpm in an attempt to obtain a highly efficient area. As this occurs, a high-frequency vibratory force by an electromagnetic force is applied to a stator, whereby an inner housing portion which holds the stator is caused to vibrate, and the whole of a motor case is vibrated by micro-amplitude high frequency waves, high-frequency vibration noise of several kHz being thereby generated. However, the motor in which the cooling water jacket is completely hollowed originally has difficulty in increasing the rigidity thereof, and in the event that a high-frequency vibratory force is applied to stator, the structural rigidity of the motor case is insufficient for the rigidity required for motor cases. In addition, in the case of the motor described in JP-A-8-19218 in which the radiation fins are provided in the hollow portion provided as the cooling water jacket and the inner housing portion and the outer housing portion are connected together by part of the radiation fins, since the radiating effect cannot be obtained when the radiation fins are made thick, the inner housing and the outer housing are connected together by thin plate-like members. Consequently, in the event that a high-frequency vibratory force is applied to the stator, the structural rigidity of the motor case is still insufficient for the structural rigidity required for motor cases, and the reduction in vibration noise level has still been difficult. Further, when an inner housing and an outer housing are connected together by radiation fins, in the event that a hollow portion residing between the inner housing and the outer housing is not increased, it becomes difficult to discharge core sand which is used as a core to form the hollow portion. Consequently, when attempting to connect the inner housing and the outer housing together by the radiation fins, there has been caused a problem that the external shape of the motor itself has to be enlarged. In this way, in the field of motors for electric vehicles, with a view to suppressing vibrations without enlarging the external shape of a motor, an increase in structural rigidity, in particular, an increase in rigidity of an inner housing which holds a stator has been desired.